Can I become a werewolf if I am a Vampire Lord?

Yep, you can. When taking on lycanthropy it will, however cure you of your Vampirism and you will no longer be a vampire lord (unless you glitch it to become a hybrid). You can become a vampire lord as many times as you'd like after curing it, but you can only become a werewolf twice (once when you are turned for the first time and once more if you cure it and wish to reaquire it).

So you can not be both at the same time?

No. There's a glitch that allows you to be a werewolf and a normal vampire at the same time, but I don't know if it works for vampire lord.

There is a glitch (You can find it on youtube. There should be a post about it here a page or two back.) that will allow you to be both a Vampire Lord (With VL form) and a werewolf at the same time.

I saw it on Youtube. But I did not find a way to become both. I became a companion but did not go further yet. I am still a Vampire Lord.